Jeffry D. Grigsby

Origin and Growth Mechanism of Authigenic Chlorite in Sandstones of the Lower Vicksburg Formation, South Texas

ABSTRACT Authigenic chlorite, occurring as grain coatings, pore fillings, and rosettes, is common in deltaic sandstones of the lower Vicksburg Formation in South Texas. Chlorite rosettes are isolated and occur late in the diagenetic history, whereas grain-coating and pore-filling chlorite occur early, predating the development of secondary porosity and the precipitation of quartz overgrowths and Fe-poor calcite. Grain-coating and pore-filling chlorite, which are interstratified with approximately 15 percent 7 A layers, are Fe-rich with a mean Fe/(Fe+Mg) ratio of 0.68. Stable-isotope data (18O averages 13.5o/oo (SMOW)) indicate that the grain-coating and pore-filling chlorite began precipitating between 20°C to 40°C within the upper 1.8 km (6000 ft) of burial. Sandstones of the lower Vicksburg Formation were deposited in a relatively shallow deltaic environment as part of a large fluvial system draining the volcanics of Sierra Madre Occidental, Mexico and Trans Pecos, Texas. Weathering of these volcanics brought large amounts of particulate Fe to the river mouth. An increase in salinity in the distributary channel complex caused Fe flocculation and deposition. During periods of high river discharge, or when coastal currents acted to concentrate particulate Fe, large amounts of particulate Fe were deposited in the delta system. This resulted in thick layers of Fe-rich clay (odinite?) around grains. During periods of low river discharge, or when coastal currents caused wide distribution of particulate Fe, less particulate Fe was deposited. It is proposed that the grain-coating and pore-filling chlorite began forming through the transformation of an Fe-rich clay precursor in the formation-water hydropressure zone following Ostwald processes. The presence of this synsedimentary Fe-rich clay, and its recrystallization to Fe-chlorite, strongly influenced fluid flow and played a large role in developing the diagenetic heterogeneity seen in lower Vicksburg sandstones today.

GeoTools: An android phone application in geology

GeoTools is an Android application that can carry out several tasks essential in geological field studies. By employing the accelerometer in the Android phone, the application turns the handset into a pocket transit compass by which users can measure directions, strike and dip of a bedding plane, or trend and plunge of a fold. The application integrates functionalities of photo taking, videotaping, audio recording, and note writing with GPS coordinates to track the location at which each datum was taken. A time-stamped file name is shared by the various types of data taken at the same location. Data collected at different locations are named in a chronological sequence. At the end of each set of operations, GeoTools also automatically generates an XML file to summarize the characteristics of data being collected corresponding to a specific location. In this way, GeoTools allows geologists to use a multimedia approach to document their field observations with a clear data organization scheme in one handy gadget.

Chemical fingerprinting in detrital ilmenite; a viable alternative in provenance research?

ABSTRACT Detrital ilmenite grains carry unique chemical fingerprints that can be used in provenance research. The elemental composition (Mn, Ti, V, Mg, Cr, Al; reported in weight percent oxide as determined by electron probe microanalysis) of homogeneous detrital ilmenite grains from felsic plutonic and volcanic, intermediate volcanic, and marie plutonic and volcanic parent rocks indicates that chemical signatures obtained in felsic and marie igneous parent rocks are retained by detrital ilmenite grains in daughter sands. Grains from marie igneous sources are enriched in TiO2 (50 ± 1%) and have a more narrow range then grains from felsic igneous sources (48 ± 2%). In addition, grains with MnO > 2.0% are most prevalent in sands from felsic rocks. Stepwise discriminan function analysis, using the oxides MnO, TiO2, V2O3, MgO, Cr2O3, and Al2O3 as the discriminating variables, found that grains from felsic and marie igneous rocks could be correctly classified 86% of the time. However, when grains from intermediate volcanic sources were added to the model, correct classification dropped to 62%. The complex chemical nature of detrital ilmenite grains from intermediate igneous sources indicates that there is much yet to learn about the composition of detrital ilmenite grains. However, if used in conjunction with other methods, chemical fingerprinting in detrital ilmenite may be a valuable addition to provenance research.

Effects of Diagenesis on Enhanced-Resolution Bulk Density Logs in Tertiary Gulf Coast Sandstones: An Example from the Lower Vicksburg Formation, McAllen Ranch Field, South Texas

Diagenetic heterogeneity strongly influences porosity and permeability distribution in sandstone reservoirs of the lower Vicksburg Formation. Enhanced-resolution bulk density logs, which use a 3-cm (1.2-in.) rather than the standard 15-cm (6-in.) sampling increment, offer important new information in understanding diagenetically complex sandstones in the subsurface. Detailed petrographic mapping and sedimentologic description of core from the Oligocene lower Vicksburg Formation of McAllen Ranch field, south Texas, have identified cement variations that result in diagenetic zones ranging in thickness from 0.1 cm (0.04 in.) to 0.9 m (3 ft). The petrographic and petrophysical properties of these diagenetic zones define three diagenetic facies that can be correlated with log esponse from the enhanced-resolution bulk density log: (1) the quartz-cemented facies, averaging 13% porosity and 0.342 md permeability, and having a recorded density greater than or equal to 2.43 g/cm3 but less than 2.48 g/cm3, (2) the chlorite-cemented facies, averaging 16.7% porosity and 0.193 md permeability, and having a recorded density less than 2.43 g/cm3, and (3) the calcite- or transitional-cemented facies, averaging 9.4% porosity and 0.04 md permeability, and having a recorded density greater than or equal to 2.48 g/cm3. Intervals cemented with different minerals are identified by the enhanced-resolution bulk density log based on the ranges in density that define each diagenetic facies. This identification of diagenetic facies not only influences the accurate determination of the net sandstone thickness and porosity of reservoirs within the lower Vicksburg Formation, but also directly controls permeability calculations. For example, classification of cement intervals from lower Vicksburg gas reservoirs using enhanced-resolution bulk density logs and porosity and permeability regressions for each of the diagenetic facies results in estimated permeability thicknesses as much as 50% greater than those calculated using standard log analysis. Because standard log analysis ignores the effects of diagenetic eterogeneity, the use of the enhanced-resolution bulk density log provides significant information that leads to more accurate reservoir and field evaluations in diagenetically complex sandstones.

Recognition and Implications of Volcanic Glass Detritus in the Fluvial Deposits of the Middle Frio Formation, South Texas

ABSTRACT Detrital volcanic glass has been identified in fluvial deposits of the middle Frio Formation of South Texas. Petrographic analysis of core samples from Seeligson and Stratton fields reveals abundant unabraded to slightly abraded, very fine sand- to silt-sized glass shards. Gamma-ray log response correlates with the presence of volcanic glass. Volcanic glass and API unit counts markedly increase through the medial third of the middle Frio and diminish toward the upper third of the middle Frio. The stratigraphic and aerial distribution of volcanic glass detritus in middle Frio sandstones and mudstones suggests that volcanic ash was deposited rapidly on the Oligocene coastal plain of South Texas. Ash was apparently also deposited in the catchment basin of fluvial systems delivering sand to the Stratton area. Given the regional biostratigraphic constraints of middle Frio fluvial deposits, the interval containing volcanic glass is regarded as representing the 30- to 34-mya eruptive event in the Trans-Pecos region.

Abstract: Outcrop-Constrained Characterization of Stratigraphic Architecture in Deltaic Gas Reservoirs, Lake Creek Unit, Texas

ABSTRACT The Lake Creek Unit of the Houston Embayment, Texas, encompassing 4.32 mi and containing 47 wells that penetrate deltaic sandstones of the Wilcox Group (lower Eocene), offers an exceptional opportunity to assess reservoir heterogeneity in gas reservoirs. Stratigraphic architecture of the G sandstone, 1 of 18 gas-condensate reservoirs ranging in depth from approximately 9,200 to 14,500 ft, was determined using wireline logs and cores. Models of deltaic architecture developed in outcrops of the analogous, deltaic Ferron Sandstone (Cretaceous), Utah, were used to constrain the subsurface interpretation. The G sandstone, approximately 300 ft thick, is one of several upward-coarsening intervals in the lower Wilcox. Mud-rich intervals that separate deltaic intervals and represent local flooding of abandoned, foundering delta lobes allow delineation of four parasequences. Each parasequence comprises genetically related depositional facies showing dimensions and spatial distribution comparable to those in outcrops of the Ferron Sandstone. The youngest parasequence is thinner and contains more abundant, marine-influenced facies than do underlying parasequences, indicating depositional landward stepping similar to that of the upper Ferron sandstones. The upper and lower parts of the parasequences are interdistributary-bay and shelf mud-rich facies, locally lignitic, and crevasse-splay and delta-destructional sandstones. They bound sand-rich intervals comprising delta-front facies locally overlain or replaced by channel-mouth-bar facies, in turn locally overlain or replaced by distributary-channel facies. Permeabilities range from about 20 md to less than 1 md but are mostly less than 1 md, the best values generally corresponding to channel-mouth-bar and distributary-channel sandstones. Deltaic sandstones and intervening bay/shelf shales result in stratified reservoirs. Similar to the Ferron Sandstone, fluid-flow barriers and baffles locally develop because of contrasting permeabilities across facies boundaries within the parasequences and the occurrence of low-permeability, mostly mud-rich intervals interbedded with the deltaic sandstones. Stratigraphic heterogeneity and low-permeability, small-drainage radii locally result in untapped and incompletely drained compartments. End_of_Record - Last_Page 807-------

Diagenetic Variability in Middle Frio Formation Gas Reservoirs (Oligocene), Seeligson and Stratton Fields, South Texas

ABSTRACT Two distinct reservoir types (type I and type II), differentiated on the basis of framework mineralogy, diagenetic history, and reservoir quality, are present in the Oligocene middle Frio Formation in Seeligson and Stratton fields, South Texas. Type I reservoir sandstones are feldspathic litharenites to litharenites having an average composition of Q30F25L45. Lithic fragments are predominantly volcanic and carbonate rock fragments. These sandstones underwent early calcite precipitation, after which secondary porosity developed (dissolution of feldspar, rock fragments, and calcite cement). Kaolinite or chlorite and pyrite postdate secondary porosity and locally influence porosity and permeability trends. Porosity averages 20% and ranges between 7% and 28%. Permeability averages 400 md and ranges between 0.007 md and 4,000 md. The large variation in porosity and permeability in these gas reservoirs is directly related to the irregular distribution of calcite cement and the development of secondary porosity. Type II reservoir sandstones, which occur in the medial third of the 2,000- to 2,500-ft thick (600- to 760-m) middle Frio Formation, are lithic arkoses to feldspathic litharenites having an average composition of Q28F31R41. Lithic fragments are predominantly volcanic rock fragments; carbonate rock fragments are rare. These sandstones contain abundant unabraded to slightly abraded glass shards and volcanic ash matrix. Devitrification of volcanic glass detritus has resulted in analcime precipitation, which precedes and overlaps in time with an early calcite cement phase similar to that found in type I reservoirs. Secondary porosity, which developed through the preferential dissolution of volcanic glass and feldspar, postdates these cementation events and has resulted in the development of moldic and intragranular porosity that can be as high as 18%. Mixed-layer illite-smectite lines much of the secondary pores. Porosimeter porosity averages 18% and ranges between 5% and 27%. Permeability averages 5.5 md and ranges between 0.005 md and 78 md. Although average porosity in type II reservoirs is similar to that in type I reservoirs, much of it is ineffective. The precipitation of calcite and analcime in intergranular pores and illite-smectite in secondary pores, combined with abundant volcanic ash matrix, results in poor communication between pores and a corresponding decrease in reservoir quality.